From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
It is essential to determine why you are air drying oak. Perhaps the most common business reason for air drying is that the lower MC after 30 to 60 days of air drying will reduce kiln drying times 50% to 75%, compared to green. On the other hand, the quality losses in air drying (which almost always are seen only after the lumber is fully kiln dried and is unstacked or even after planing) can be quite large (10% of the lumber抯 value is often seen). With oak, the lumber must not be dried too quickly. Further, the lumber should be protected from direct rainfall or moisture from melting snow.
The rate at which lumber dries is determined by the relative humidity, temperature, and air flow past the lumber. These values may be the values reported by the Weather Channel, but are more likely modified by the local climate around your pile of lumber. Grass or trees may reduce air flow and also lower temperatures and raise humidities, for example. Therefore, it is important to evaluate your air drying yard to make sure that you have good air flow, warm temperatures, and not real low RHs. This is why we have suggestions about keeping the lumber piles at least 12 inches off the ground with an opening at least 6?high to allow air underneath to escape, and keeping at least 24 inches between piles, edge to edge.
Most quality loss in air drying oak is a result of rewetting by rain and/or excessively fast drying (hot, dry, windy days with lots of sunlight). These undesired drying conditions can be avoided or moderated by drying the lumber in an open shed (see next section).
In order to avoid putting warp into the lumber, the foundation in air drying must be perfectly flat. Permanent foundations are hard to repair; movable foundations require constant attention. The foundation is usually wood and so must be treated to avoid decay. The open space under the pile to the outside must be at least 6 inches to allow for adequate circulation under the pile and allow for moisture air to be carried away. The base for an air yard must be solid. A stack of lumber that is 5?wide x 12?long and 50 layers high (oftentimes considered to be two stacks) will be roughly 4 MBF and weigh as much as 24,000 pounds. The surface also must be well drained, so that any water that is evaporated by warm, dry air is water from the lumber and not from the soil.
As a first step in reducing quality loss, the topmost pile should be covered with a roof to shed the rain and protect the top layer from sunlight. Often the top layer in a pile is dunnage lumber. Tar paper or other water resistant material can be incorporated into the pile to provide rain protection. Commonly, plastic fabric resembling burlap is used to cover the piles on the top and even on the ends or edges to reduce air flow, if required.
One important fact is that the drier the lumber, the higher the risk of quality loss when it rains. In other words: DO NOT OVER-AIR-DRY. Move lumber into the kiln promptly.
Although many yards have two packs of lumber, edge to edge with a 2?gap between the two packs, if a bit slower air drying is desired, such as with oak, then three packs in width is common, with the same 2-foot spacing. It is important to note that when actually loading packs of lumber into the yard, try and fill a section to the designed width and height rather than having partially filled areas, which would lead to uneven and too rapid drying.
If flat lumber is desired, there should be a support, called a bolster, or 4x4, at every location where there is a row of stickers (usually 24?spacing). Note that it is advisable to put a stick down on the top of the bolster so that the lumber contacts only the narrow sticker and not the wide timber. Alternately, put in several wide grooves into the bolster to allow air flow.
One factor that causes quality loss is rain on the lumber. For this reason, with valuable wood, it is advisable to have a roof to cover the top stack of lumber. Although the side of the stack will still be wet from rain, the top will be protected. Roofs can be made of old lumber, fiberglass or any other appropriate material. It is very important that the roof be secured to the lumber pack so it will not blow off during strong wind and potentially cause a lot of damage to buildings, vehicles or people nearby. The roof is usually put on top of stickers rather than directly on top of the top layer of lumber.
It is important that the pile bottoms be open for the moist air to escape. This means that weeds and grasses growing in the air yard cannot be tolerated.
One reason we are concerned about even drying with a pack, especially even from top to bottom, is that when lumber goes into the kiln, the wettest lumber controls drying time. If a poor practice in the yard leads to wetter lumber near the bottom of the stacks, this means longer kiln drying time and higher kiln drying costs. I have seen in a few cases where a lift-truck is used to move lumber, that about halfway through the AD cycle, the lumber packs are repositioned, with the bottom packs going to the top and the top to the bottom. This provides more even drying and can save 10% in kiln drying time, energy and cost.
Sometimes an AD yard dries too fast. One way to slow down air drying, such as during the springtime when winds are strong and humidities are low, is to cover one side of the pack with a plastic burlap material such as Shade-Dri. Be cautious that drying is not slowed too much as mold and other stains can develop.
Insects sometimes can get into the lumber. One common insect is the ambrosia beetle. Wood debris in the yard assists or encourages these insects to reproduce. Keep yards clean.
As a final thought, oak lumber that has been on the AD yard for much more than a couple of months can become so dry that wetting by rain and then drying again in a few days will create checks, cracks and warp. Long drying times also increase the risk of insect damage and surface molds. The admonition here is to avoid over air-drying. Once the lumber reaches about 30% MC, consider moving the lumber into the kiln or storage shed rather than leaving it outside.
Many details are in the Air Drying of Lumber book.